1. Field of the Invention
The invention concerns the retransmission of digital signals received jointly by cable or RF transmission in a domestic setting. It applies primarily to digital terrestrial television and retransmission to mobile televisions.
2. Description of Related Art
Digital terrestrial television signals are transmitted primarily by way of microwave radio systems In most countries. But some countries also use cable distribution systems and therefore want to multiplex the channels from their cable network and those from the RF channels to increase the mobile television penetration rates in homes, For example Belgium, 95% cabled, is one of these countries potentially interested in joint retransmission for mobile television.
Currently to receive mobile television under good conditions on one's portable or mobile terminal at home, a television channel retransmitter previously connected to the home's roof antenna is used, This retransmitter is installed in the home. Thus only the television channel, received by the roof antenna, will be amplified and then transmitted at the same frequency for improved interior reception, This involves an isofrequency retransmission,
Other than in the case of joint distribution by radio relay channel and by cable, the TV channel frequency is not necessarily the same by cable or by radio relay channel Consequently, there are disturbances that hinder the proper functioning of the terrestrial broadcasting network.
Seen from the mobile TV terminal, this variance can be compared to the Doppler effect. However, it should be noted that unlike the Doppler effect, the variation in time of this variance will be practically nil because the reception is in fixed mode.
The state of the art, represented by FIG. 1, uses an isofrequency retransmitter locked to the frequency of the television channel to retransmit.
This retransmitter is based on a double frequency transposition on the uplink and the downlink such that the signal is transmitted on the frequency of the memorised tuner channel, The frequency offset related to the double change of upstream and then downstream frequency auto-compensates by adding and then subtracting the same frequency deviation. The different selective bandwidth filters used to isolate the channel to be retransmitted have severe out-of-band rejection constraints that tolerate little frequency offset, For example, FIG. 3 presents the out-of-channel transmission spectral mask of a relay transmitter on the DVB_H standard for a channel frequency band of 8 MHz.
Therefore, on the input terminal of this retransmitter, represented in FIG. 1, the Rfin signal is transmitted by cable, A SW1 switch in the first position is used to provide a direct connection between this terminal, a tuner T element, then a demodulator D and then a microprocessor P so as to determine the frequency of the transmission channel. A transmitting frequency memorised in the memory M corresponds to this signal channel received, Thus the microprocessor sends a control signal to the first local oscillator OL1, which allows a transposition at this memorised frequency.
Since the channel has been determined, the switch SW1 moves to the second position and the signal is transposed twice in an uplink and a downlink by the M1-M4 mixers combined with the oscillators OL1 and OL2, filtered by the different RF and IF filters, F1-F5, and amplified by the amplifiers A1 and A2 with variable gain control CAG controlled by the processor. The signal transposed to the memorised transmitting frequency and corresponding to the input channel is transmitted on the output antenna, A safety switch SW2 controlled by the microprocessor can be used to interrupt the transmission when errors are detected.
For example, by assuming an oscillator TCXO of 1 ppm of frequency drift and a frequency change of 100 MHz, drawing 2 shows an estimate of the maximum Doppler frequency deviation for a mobile television signal transmitted on the DVB_H standard and accepted by a terminal equipped with the latest generation digital demodulator.
In this FIG. 2, it should be noted that for a DVB_H type transmission as represented by the curve C1, based on a maximum frequency derivation value of 191 Hz, corresponding to a signal-to-noise ratio of 22 dB, the demodulator blacks out and no longer operates.
By way of comparison, a Rayleigh type fixed propagation channel would only require a signal-to-noise ratio of 11.2 dB, at the same error rate,
The choice of the Doppler frequency value of 100 Hz, which corresponds approximately to a signal-to-noise ratio of 19 db, is therefore adopted as the correction limit for this Doppler effect.
The choice of the 100 Hz value takes into account an additional implementation margin.
Consequently, when the frequency change is less than or equal to 100 MHz, the receiver can diversity receive 2 DVB_H signals whose channel frequency will be offset by a frequency less than 100 Hz and therefore correctly managed by most receivers.
However, if the frequency change is greater than 100 MHz, a traditional frequency transposition approach would require either a precise OCXO type reference oscillator or an automatic frequency control system (AFC). But this oscillator or system are bulky, quite expensive and incompatible with mass production.
The patent EP 1744471 describes another isofrequency retransmission system and the associated process. This patent deals with isofrequency retransmission systems also called “gap filler” systems, whose main function is to receive the signal from a main transmitter on a receiving antenna and then to retransmit this signal at the same frequency to the zone to be covered by a transmitting antenna. This patent includes a retransmission technique by subtraction, which has the effect of cancelling the coupling echo caused by the transmitting antenna and the guarantee of improved signal retransmission conditions.
Even though this technique is an isofrequency retransmission technique, it can be used to correct deformations related to coupling and not those related to a frequency offset comparable to a Doppler effect.